Oscillators comprising a LC-oscillating circuit are used e.g. for proximity switches in which the approach of a driving blade produces a damping of the oscillating output signal of the oscillator. The amplitude of the output signal of the oscillator decreases and drives, e.g. a level detector, which delivers a useful output signal from the proximity switch.
The principal disadvantage of conventional inductive proximity switches is their relatively limited switching distance. An increase in the switchihg distance is hindered by the temperature dependence of the combination of the oscillator circuit, and the oscillating circuit, which with an increasing switching distance gives rise to a normally unacceptable temperature coefficient proportional to switching distance.
FIG. 1a shows a behaviour of the relative quality factor Q/Qo of an oscillating circuit as a function of the distance d (switching distance) of the driving blade of an inductive proximity switch. It may be seen that with an increasing switching distance d, the useful change of the relative quality factor Q/Qo of the oscillating circuit sinks rapidly to a very small value. If the normal working range is such that for a given switching distance the relative quality factor has a predetermined value of 50%, the curve of FIG. 1a shows that the relative quality factor drops to about 3% if the switching distance is increased three times.
The influence of the ambient temperature upon the relative quality factor of the oscillating circuit is indicated in FIG. 1b which shows that the ratio Q/Qo decreases with increasing temperature. A comparison with FIG. 1a indicates that for greater switching distances, the influence of temperature upon Q/Qo rapidly becomes more important than the change resulting from switching distance of the driving blade.
It is therefore an object of the present invention to provide a process capable of greatly reducing the temperature dependence of the oscillating circuit so that proximity switches having a switching distance several times greater than the conventional ones may be used.